1. Technical Field
The present disclosure relates to a magnetoresistive sensor and the method for its manufacture.
2. Detailed Description
As is known, magnetoresistive sensors exploit the capacity of appropriate ferromagnetic materials (referred to as magnetoresistive materials, for example the material known by the name of “permalloy” constituted by an iron-nickel (Fe—Ni) alloy) for modifying their own resistance in the presence of an external magnetic field.
Currently, magnetoresistive sensors are obtained from strips of magnetoresistive material. During fabrication, the strip of magnetoresistive material is magnetized so as to have a preferential magnetization in a preset direction, for example, the longitudinal direction of the strip.
In the absence of external magnetic fields, the magnetization maintains the direction imposed, and the strip has a maximum resistance. In the presence of external magnetic fields having a direction different from a preferential magnetization direction, the strip magnetization changes, as well as its resistance, as explained hereinafter with reference to FIG. 1.
In FIG. 1, a magnetoresistor 1 is formed by a strip of magnetoresistive material having a longitudinal direction parallel to axis X. The magnetoresistor 1 is traversed by a current I flowing in the longitudinal direction of the strip. An external magnetic field Hy is directed parallel to the axis Y and causes rotation of the magnetization M through an angle α with respect to the current I. In this case:R=Rmin+ΔR cos2αwhere Rmin is the resistance of the magnetoresistor for a magnetization M parallel to the axis Y (very high external magnetic field Hy), and ΔR is the resistance difference Rmax−Rmin, where Rmax is the resistance for a magnetization oriented parallel to the direction X. For the permalloy, the maximum ratio ΔR/R is of the order of 2-3%.
By setting
                    sin        2            ⁢      α        =                                        Hy            2                                Ho            2                          ⁢                                  ⁢        for        ⁢                                  ⁢        Hy            ≤      Ho        and                    sin        2            ⁢      α        =                  1        ⁢                                  ⁢        for        ⁢                                  ⁢        Hy            ≥      Ho      where Ho is a parameter that depends upon the material and upon the geometry of the strip 1, we have:
                    R        =                                            R              min                        +                          Δ              ⁢                                                          ⁢                              R                [                                  1                  -                                                            (                                              Hy                        Ho                                            )                                        2                                                  ]                            ⁢                                                          ⁢              for              ⁢                                                          ⁢              Hy                                ≤          Ho                                    (        1        )            
FIG. 2 shows with a dashed line the plot of the resistance R resulting from Eq. (1) (curve A).
In order to linearize the plot of the resistance R at least in an operative portion of the curve, it is moreover known to form, above the strip of magnetoresistive material, transverse strips 2 (referred to as “barber poles”) of conductive material (for example aluminum), arranged at a constant distance from one another and with an inclination of 45° with respect to the direction X, as shown in FIG. 3.
In this situation, the direction of the current I changes, but not the magnetization. Consequently, Eq. (1) becomes:
                    R        =                                            R              min                        +                                                            Δ                  ⁢                                                                          ⁢                  R                                2                            ±                              Δ                ⁢                                                                  ⁢                                                      R                    ⁡                                          (                                              Hy                        Ho                                            )                                                        [                                      1                    -                                                                  (                                                  Hy                          Ho                                                )                                            2                                                        ]                                ⁢                                                                  ⁢                for                ⁢                                                                  ⁢                Hy                                              ≤          Ho                                    (        2        )            which has a linear characteristic in the neighborhood of the point Hy/Ho=0, as shown by the curve B with solid line of FIG. 2. The sign ± in Eq. (2) depends upon the orientation of the barber poles 2 (±45°).
FIG. 4 shows a magnetoresistive sensor 9 including four magnetoresistors 1 having barber poles 2 arranged in an alternating way and connected so as to form a Wheatstone bridge. In detail, the two magnetoresistors 1a, 1b belonging to each branch 3, 4 have barber poles 2 oriented in an opposite way, are arranged diametrally opposite, and are connected in series between two terminals 5, 6. A biasing voltage Vb is applied to the terminals 5, 6.
Trimmer resistors can be connected in series to each branch 3, 4, in a way not shown, so as to have a zero output in the absence of an external magnetic field oriented parallel to the direction of detection (here the field Hx).
The output voltage Vo existing between the intermediate nodes 7, 8 of the branches 3, 4 is thus correlated to the existing external magnetic field Hx. In fact, an external magnetic field Hx causes an increase in the resistivity of the magnetoresistors 1a having barber poles 2 oriented in a first direction and a corresponding reduction in the resistivity of the other magnetoresistors 1b. Consequently, any variation of resistance due to an external field causes a corresponding linear variation of the output voltage Vo, and thus a linear relationship between them exits.
Given the high sensitivity of the magnetoresistive sensors of the type indicated, use thereof has been recently proposed for electronic compasses in navigation systems. In this case, the external field to be detected is the Earth's magnetic field. To a first approximation, the Earth's magnetic field can be considered parallel to the surface of the Earth and the compass uses two sensors sensitive to the two directions of the plane that is locally tangential to the surface of the Earth. However, since the inclination of the compass with respect to the tangential plane entails reading errors, the compass may have three sensors, each having a sensitive axis oriented according to the three spatial axes X, Y, Z, in order to correct the reading errors.
To this aim, the three sensors are arranged such that they are rotated through 90° with respect to one another. Whereas building a sensor sensitive to fields oriented along two directions does not create any difficulty, since they lie in a same plane, the detection of the third direction is critical, since it requires the provision of a third sensor arranged in a plane perpendicular to the first two sensors. In fact, in this case, the assembly operations are much more complex and the end device is much more costly.